Internal combustion engines may use variable cam timing (VCT) to improve fuel economy and emissions performance of a vehicle. One method of variable cam timing uses an Oil Pressure Actuated device (OPA), such as a vane type cam phaser. The phaser may be controlled by an electromechanically actuated spool valve that directs oil flow to one side or the other of the vane. The performance of this device is thus dependent on oil pressure, which can be a function of engine speed and leakage through various oil subsystems. Therefore, the Oil Pressure Actuated device may have unacceptable performance at low engine speeds or when hydraulic subsystems exhibit high oil leakage.
One example VCT system includes a vane type actuator as well as an optional biasing spring to hold the cam timing in a base position whenever insufficient oil pressure is available to control position of the actuator via the spool valve. For example, the base timing may be a fully retarded timing desired for engine starting, since sufficient oil pressure is typically unavailable during engine starting operation.
The inventors herein have recognized that OPA cam phasers may be particularly prone to oil leakage and slow response time when held at base timing by hydraulic actuation force controlled by the spool valve. Specifically, under this condition, the spool valve is positioned in a full retarding actuation position, as the full retarding actuation position is often the default (de-energized) state of the spool valve. For the example of a cam-fed oil pressure actuated system, oil leakage may occur between the advance and retard oil passages in the cam journal bearing due to the pressure differential between the two passages. In the base position, oil fully pressurizes the base position oil passages in the cam journal. Because the oil control valve may have one port fully pressurized and another open to tank (atmosphere) in the de-energized position, too much oil may flow across the small radial clearance and the lateral seal land distance between the two passages in the cam journal (flow from high to low pressure). Oil flows through the oil passages and out of the drain port in the spool valve may reduce main galley oil pressure and thus create a significant oil pressure drop in the system.
As such, one approach to address the above issues includes a method of controlling an engine, the engine including a hydraulically actuated variable cylinder valve actuator coupled to a cylinder valve of the engine. The actuator is controlled by a hydraulic valve adjustable among a plurality of ranges including a first range generating a hydraulic force in a first direction on the actuator toward a first end position, a second range generating a hydraulic force in a second, opposite direction on the actuator toward a second, opposite end, position, and a neutral range between the first and second ranges. The method comprises during selected conditions and when the actuator is held at the first end position by a biasing spring, adjusting the valve to within the first range and closer to the neutral range than to a full actuation boundary of the first range. In one example, the variable cylinder valve actuator may include a variable cam timing system that further includes oil passages integrated into a cam journal. In another example, the hydraulically actuated variable cylinder valve actuator includes a variable cam timing vane type actuator having a biasing spring biasing the actuator toward a retarded cam timing base position.
In this way, when operating with the variable cam timing actuator at base timing, such as during idle conditions, the valve can be adjusted away from the full actuation boundary of the first range, thus reducing oil leakage, such as across a cam journal. This reduced leakage can thus increase main galley pressure and improve the performance of other hydraulic systems in the engine. Further, such positioning of the valve is acceptable as the actuator can still be maintained at base timing. In addition, when it is desired to move the cam timing away from base timing, a faster response (e.g., movement in the advance direction) can be achieved because there is less retard pressure to overcome.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.